Investigation has shown that heating either potassium or sodium plumbite-treated cotton or other cellulosics in fabric or sheet form while the treated cellulosic is in contact with a glass or ceramic and the heating taking place in a limited oxygen atmosphere such as an oven at about 600.degree. to 700.degree.C. for about 60 minutes produces a hard glasslike, electrically non-conductive film. The sodium plumbite is preferred mainly because of its availability.
Further investigation has revealed that a thin layer of metallic aluminum against the exposed surface of the plumbite-treated cellulosic in the above procedure results in a dull grey, glassy hard film that is electrically conductive, and because of its hardness is resistant to damage by cutting or abrading. The thin layer of aluminum may be provided as metallic foil or as particles for use in the process of this invention.
Investigation has shown that the assembly of aluminum foil or powder over plumbite-treated cellulosic which is over glass or porcelain, must be placed in the oven before the oven temperature exceeds about 200.degree.C.
Copper and tin did not act in a manner similar to the aluminum in the process. Some variation in electrical conductivity of the coating can be achieved by varying the lead content of the plumbite-treated cellulose or by varying the amount of aluminum. As an example, a more conductive film was formed by use of aluminum as foil than as powder. A film made using plumbite-treated cellulose with 8% lead and aluminum foil was more conductive (zero ohm resistance) than a film made using plumbite-treated cellulose with 24% lead and identical aluminum foil (50 ohms resistance). The conductivity of the film can also be varied by treating the good conducting film as initially formed with a strong acid such as concentrated nitric acid. Immersing the film in concentrated nitric acid for a period of 8 hrs, lowers the conductivity such that the resistance is 2600 ohms. Measurement of conductivity of the films of the invention was made by measuring the electrical resistance (in ohms) between two points 1 inch apart on the film. The resistance of the film could be varied so that the conductivity was either nearly perfect (zero ohms) or moderate (several thousand ohms) per inch of surface measured between points one inch apart on a film 0.0015 inches thick. Anyone skilled in the art of electrical conductors and the use of same will understand the inverse relationship of conductivity and resistance.
Treatment of the cellulosic material used an alkaline aqueous solution of sodium plumbite prepared by saturating 12% sodium hydroxide in water with lead monoxide powder and allowing it to stand for 24 hours prior to filtration. This procedure is not a part of the process of this patent and is known from prior art.
Aluminum foil alone does not adhere to the glass when heated to the temperature of the process of this invention.
Electron emission spectroscopy revealed the surface of the conducting film to a depth of 100 Angstroms contains aluminum, lead, silicon, oxygen, and carbon.
Prior art teaches that aluminum may be added to glass as the oxide during glass formulation. Prior art also teaches that metalic aluminum may be coated by deposition to form a conductive coating. Prior art, on the other hand, does not teach the formation of a conductive coating by fusing a glassy layer to the mother of glass by using the process or ingredients of the present invention.